The present invention relates to a tire air pressure sensor used for vehicles such as automotive vehicles, and a tire air pressure measuring apparatus incorporating the sensor.
Japanese unexamined patent application No. Sho. 55-31975 published Mar. 6, 1980 teaches an extraordinary-pressure sensor which includes a pressure-responsive switch which is opened and closed when the tire air pressure is normal and abnormal, respectively. When the tire pressure is to be sensed, a signal transceiver is operated, thereby producing an electromagnetic signal having a predetermined frequency which is then transmitted via an antenna to a coil antenna. The coil receives the transmitted signal and outputs it to a crystal resonator which has a resonant frequency equal to the frequency of the received signal thereby causing the resonator to resonate when the pressure-responsive switch is open. Then the transceiver stops transmitting the electromagnetic signal. For some interval after the end of transmission of the electromagnetic signal, the crystal resonator continues to resonate, thereby producing an electromagnetic signal having the same frequency as the resonant frequency and transmitting the signal via the coil toward the antenna. The signal recieved by the antenna is supplied to the receiver unit of the transceiver system, thereby displaying that the tire air pressure is normal. On the other hand, when the switch is closed, which implies that the tire pressure is abnormal, the resonator does not resonate because the resonator is short-circuited and therefore not energized. A plurality of such resonators having different resonant frequencies can be provided together with a transceiver system which can produce corresponding frequencies in order to sense different air pressures.